Filters with parallel-coupled resonators in microstrip or strip-line are known in the art, e.g., Microwave Filters, Impedance-Matching Networks, and Coupling Structures, George I. Matthaei et al., Artech House, 1980, at Section 8.09, pages 472-477. An exemplary parallel-coupled resonator filter 10 is shown in FIG. 1. The filter includes a dielectric substrate having opposed planar surfaces, with a ground plane layer on a bottom surface, and input/output (I/O) ports 14, 16. A conductor strip 14A is formed on the upper surface of the substrate to connect to the I/O port 14. A conductor strip 16A is formed on the upper surface of the substrate to connect to the I/O port 16. Microwave energy is coupled between the I/O ports by a series of conductive strips 18-1, 18-2 . . . 18-7 defining a series of spaced resonators on the upper surface. The resonators are staggered along a diagonal 20.
The parallel-coupled resonator filter is often placed in a channel in a conductive housing structure, in which unwanted waveguide modes can propagate due to the relatively large channel width needed to accommodate the width of the filter.
A band pass filter circuit for microwave frequencies is described, comprising a plurality of parallel-coupled resonators formed in a planar transmission line medium, including coupling between alternate resonators in the form of transmission line gaps.